This invention relates to the use in smectic display cells of anthraquinone dyes having strong absorption registering with the helium neon laser emission line at 633 nm.
Dichroic anthraquinone dyes having previously been used in dyed cholesteric nematic phase change (DCNPC) display cells but their role in DCNPC cells is entirely different from that in a smectic cell. In a DCNPC cell the dye is being used to provide the contrast of color that renders the display visible. For this purpose it must have a relatively high order parameter to produce adequate contrast in the display. In a smectic cell the dye is not used to render the display visible, but is used instead to absorb electromagnetic radiant energy to produce local heating of the smectic layer. For this purpose it will be apparent that order parameter is relatively unimportant, as also is the color of the dye, so long as it is strongly absorbing at the wavelength of the radiant energy. In fact in this instance a broad absorption band tends to be undesirable because it will absorb light that would otherwise usefully contribute to the visibility of the display. Other criteria for the dye are that it shall be chemically, photochemically, and thermally stable, and, on account of the thinness of the liquid crystal layer of a typical smectic cell, that it shall have a high extinction coefficient at the wavelength of the radiant energy.